The present invention relates to a process for forming a fluorescent screen, and more particularly to a process for forming a fluorescent screen suitable for the cathode ray tube.
The inside surface of the face plate of a cathode ray tube is coated with three kinds of phosphors each emitting red, green and blue in a dot or stripe pattern. According to the conventional process, such phosphor coating layers are formed as follows: First of all, a layer of first phosphor, for example, a mixture of green-emitting phosphor and photosensitive resin, is formed on the inside surface of the face plate. As the photosensitive resin, a mixture of polyvinyl alcohol and ammonium dichromate is usually used. The layer is formed usually by coating the inside surface of the face plate with a mixture of a solution of photosensitive resin with the phosphor, followed by drying of the coating. Then, the resulting layer is irradiated with ultraviolet rays through a shadow mask. The positions to be irradiated by the ultraviolet rays should correspond to the positions which electron beams are to hit to make the phosphor undergo emission, that is, the positions to which the phosphor is to be fixed. The photosensitive resin at the irradiated positions are insolubilized, and the layer at these positions is entirely insolubilized thereby. Then, the layer is washed with a solvent, normally water, to remove other parts of the layer by dissolution while retaining only the insolubilized parts obtained by the ultraviolet irradiation. Then, similar operations are carried out with another mixture of second phosphor, for example, blue-emitting phosphor, with the photosensitive resin, and then with other mixture of third phosphor, for example, red-emitting phosphor, with the photosensitive resin.
As is obvious from the foregoing, the process for forming a fluorescent screen for a cathode ray tube is complicated and requires a plurality of wet coating operations and repetitions of water washing and drying operations. Thus, simplification of the process has been keenly desirable.
To improve the said problem of the prior art, some of the present inventors proposed a process for forming a pattern of given phosphor by providing a thin layer of photosensitive substance that can turn tacky by light irradiation, for example, aromatic diazonium satl, onto the surface of a substrate, then exposing the thin layer to light through a shadow mask, thereby tackifying the pattern parts destined for the phosphor, and depositing phosphor particles onto the tackified pattern parts by adhesion (U.S. Pat. No. 4,273,842).
Another process was proposed for producing black matrix or black stripes by forming a fluorescent screen for a cathode ray tube according to the foregoing process, and then depositing black powder such as carbon powder, etc. onto the marginal parts of the phosphor pattern. However, in the production of a fluorescent screen for a cathode ray tube according to the foregoing process, such phenomena sometimes occur that, when a pattern of red-emitting phosphor is formed after the formation of a pattern of blue-emitting phosphor, the blue emission from the pattern of blue-emitting phosphor is contaminated with some red emission. Generally, there has been such a problem that the individual zones of three kinds of phosphors, i.e., blue-emitting, green-emitting and red-emitting phosphors, are liable to be contaminated with the phosphors emitting other color, with the result that the so-called color contamination phenomena are likely to appear, lowering the color purity.
In the case of using black powder, the individual zones of phosphors are often contaminated with black powder, lowering the brightness of fluorescent screen. This problem can be solved by providing a longer time in contacting a given phosphor with the light-exposed thin film, that is, by continuing the contact of the powder for a time long enough to saturate the light-exposed parts with the powder. However, provision of such a longer time only for one step is not preferable from the commercial viewpoint.
To improve such a problem, some of the present inventors proposed another process of using fixing powder (Japanese Laid-open Patent Application No. 32332/80), in which, after coating of phosphor powder, a powdery solid substance capable of forming a water-insoluble or sparingly water-soluble substance through reaction with photolytic products of a photosensitive substance is brought in contact with the coating layer of the phosphor powder. In the case of using, for example, an aromatic diazonium chloride-zinc chloride double salt as a photosensitive substance where zinc chloride is its photolytic products, calcium hydroxide, sodium hydrogen carbonate, sodium carbonate, etc. are used as a fixing powder. Deposition of the desired amount of phosphor powder onto the light-exposed parts (the tackified parts) of the thin layer takes only a short time, but saturation of the light-exposed parts with the deposited phosphor takes a longer time so that they may not be contaminated with other phosphor powder. That is, the proposed process can solve the problem of color contamination by operation of short duration.
However, the proposed process has brought about another problem. If the excess fixing powder is not completely removed from the light-unexposed parts of the thin layer and if even a very small amount of the fixing powder remains thereon, and if there is the fixing powder on the second, light-exposed parts, the thin layer turns tacky by light exposure and at the same time undergoes fixation. Accordingly, deposition of the second phosphor powder onto such parts cannot be carried out, though the phosphor is not deposited onto a whole dot of the phosphor pattern. In other words, the amount of phosphor to be deposited is extremely reduced, lowering the brightness of dot. Thus, drying and preservation of fixing powder, control of working atmosphere, removal of fixing powder, etc. must be carefully carried out. Otherwise, the product yield is inevitably lowered.